I've never understood the reasoning behind using the header when you only want the info on the first page. Headers & footers are specifically for repeating information. But whatever the reasoning, Unlike other word processors I've used, Pages doesn't automatically suppress the headers & footers on the first page of a section. If you entered data in the header or footer on the first page, that will be displayed & suppressed on the remainder. To suppress it on the first page, the information must be entered on the second or later page.
But there are other ways to do what you wanted. Personally, I'd use a text box but you could also use a layout break setting the columns to one before the break & two after. Both of this options let you "recover" the space taken up by the header on the following pages.